This invention relates to hydraulic systems.
The invention is more particularly concerned with hydraulic lift systems.
Hydraulic systems are often used in applications where people need to be lifted, such as in lifts and ambulance entry platforms. When hydraulic power is supplied to or from the actuator in such systems there can be a very sudden movement, which is disconcerting to the person being lifted. The high initial acceleration of hydraulic lifts can also be a problem where delicate goods are being lifted. It is possible to provide a hydraulic system with a soft start by use of a spool valve and a proportional solenoid. The solenoid is arranged to open or close the spool valve slowly so that hydraulic power supplied to or from the actuator is gradually increased or decreased. This arrangement can work effectively but has two disadvantages. First, the high cost of proportional solenoids and spool valves make them unsuitable for low cost applications. Second, they are unsuitable for applications where a load needs to be held, because their design means that they are inherently leaky.